The dreaded M1 Garand "thumb" occurs when your thumb gets in the way of the bolt while its chambering a round. This happens after the enbloc clip with ammunition has been inserted into the magazine. Without proper techniques your thumb will get smashed in the chamber. Ouch is a polite version of the words that will come flying from your mouth after this happens to you.

Watch that thumb or you may be the next victim of Garand "thumb". You have been warned.